1. Field of the Invention
The present invention relates to a CO removal catalyst of purifying a modified gas mainly composed of hydrogen containing carbon monoxide (hereinafter referred to as “CO”) to provide a high purity hydrogen gas, a method of producing a CO removal catalyst, a hydrogen purifying device and a fuel cell system.
2. Related Art of the Invention
As a hydrogen source for fuel cell there is used a modified gas obtained by the modification of hydrocarbon, alcohol, ether, etc. However, a solid polymer type fuel cell which operates at a temperature as low as not higher than 100° C. is subject to poisoning of Pt catalyst used in the electrode of the fuel cell by CO contained in the modified gas. When the Pt catalyst is poisoned, the reaction of hydrogen is inhibited, causing a remarkable drop of the electricity-generation efficiency of the fuel cell. It is therefore necessary that CO be removed to a concentration of not higher than 100 ppm, preferably not higher than 10 ppm by using a hydrogen purifying device.
In general, in order to remove CO, CO and water vapor are subjected to aqueous gas shift reaction (CO modification reaction) at a CO modifying portion comprising a CO modification catalyst in the hydrogen purifying device so that they are converted to carbon dioxide and hydrogen to reduce CO concentration to a range of from thousand parts per million to about 1%.
Thereafter, a slightly amount of air is used to add oxygen to the modified gas. With a CO selective oxidation catalyst, CO is then removed to a level of several parts per million, where the fuel cell cannot be adversely affected. In order to remove CO thoroughly, it is necessary that oxygen be added in an amount of from about 1 to 3 times CO concentration. However, hydrogen, too, is consumed according to the amount of oxygen. When CO concentration is high, the amount of oxygen to be added increases, causing the rise of the consumed amount of hydrogen and hence causing a drastic drop of the efficiency of the entire device.
Accordingly, it is necessary that CO concentration in the CO modifying portion comprising a CO modification catalyst be thoroughly reduced.
When used in ordinary houses, the device must be frequently actuated or suspended. Therefore, the device is required to have resistance to heat shock or entrance of air.
Referring to CO modification catalyst, as a low temperature CO modification catalyst there has been heretofore used a copper-zinc-based catalyst or copper-chromium-based catalyst, which can be used at a temperature of from 150° C. to 300° C. As a high temperature CO modification catalyst there has been used an iron-chromium-based catalyst, which performs at a temperature of not lower than 300° C. Referring to the use of these CO modification catalysts, the low temperature CO modification catalyst has been used singly or in combination with the high temperature CO modification catalyst depending on purposes such as chemical plant and hydrogen generator for fuel cell.
When a copper-based low temperature CO modification catalyst is mainly used, a very high catalyst activity can be obtained. However, it is necessary that such a copper-based low temperature CO modification catalyst be subjected to reduction before use so that it is activated. Further, in order to prevent the catalyst from being heated to a temperature beyond its heat resistance due to the heat generation during the activation, it is necessary that the catalyst be processed in a long period of time while the supplied amount of the reducing gas being adjusted. Moreover, since the CO modification catalyst which has once been activated can be reoxidized and deteriorated when mixed with oxygen during the suspension of the device or other occasions, some countermeasures such as antioxidation treatment are needed. Further, since the low temperature CO modification catalyst has a low heat resistance and thus cannot be rapidly heated during the starting of the device, some countermeasures involving gradual heating or the like are needed.
On the other hand, when only a high temperature CO modification catalyst is used, the catalyst be easily heated during starting because the catalyst has a high heat resistance and thus can be heated to a temperature as high as about 500° C. without any problem. However, CO modification reaction is an equilibrium reaction that can difficultly proceed toward the reduction of CO concentration at high temperatures. Thus, when the high temperature CO modification catalyst, which performs only at high temperatures, is used, CO concentration can be difficultly reduced to not higher than 1%. Therefore, the purification efficiency at a CO purifying portion provided after this stage can be reduced.
In order to avoid the aforementioned shortcomings of the related art catalysts, noble metal modification catalysts mainly composed of Pt, which is excellent in heat resistance as well as in oxidation resistance, having a higher low-temperature activity than iron-chromium catalyst are disclosed (as in JP-A-2000-302410, for instance).
The entire disclosure of JP-A-2000-302410 is incorporated herein by reference in its entirety.
More particularly, JP-A-2000-302410 discloses the use of a pelletized cerium-zirconium composite oxide as a noble metal modification catalyst.
Further, a noble metal catalyst having Pt and optionally Re and Pt supported on a zirconia carrier is disclosed (see Japanese Patent No. 3215680, for instance).
The disclosure of Japanese Patent No. 3215680 is incorporated herein by reference in its entirety.
Moreover, a noble metal catalyst having Pt supported on a carrier having a base point such as titania on which a rare earth element such as La and Ce has been supported is disclosed (see WO00/48261 (PCT application), for instance).
The disclosure of WO00/48261 (PCT application) is incorporated herein by reference in its entirety.
Besides the aforementioned disclosures, similar noble metal catalysts are disclosed (see JP-A-2001-316682 and JP-A-2001-322803, for instance).
The disclosure of JP-A-2001-316682 and JP-A-2001-322803 are incorporated herein by reference in its entirety.
Further, it is disclosed that as a binder for pelletization of rutile titania there is preferably used zirconia sol (see JP-A-2002-128507 and JP-A-2002-95966, for instance).
The disclosure of JP-A-2002-128507 and JP-A-2002-95966 are incorporated herein by reference in its entirety.
The noble metal catalysts disclosed in the aforementioned patents are worked into tablets or spherical forms which are then packed in the reactor. Referring to the method of preparing the catalyst, a metal oxide which is stable at high temperatures or in a reducing atmosphere such as alumina, zirconia and titania is formed into tablets or spherical forms. An additive such as alkali and rare earth elements is supported on the carrier thus formed. Finally, a noble metal such as Pt is supported on the carrier.
However, the aforementioned related art catalysts are disadvantageous in that the additive (alkali, alkaline earth, transition metal and rare earth elements) cannot be supported on the molded alumina, zirconia or titania (catalyst carrier) up to the interior thereof and only the noble metal supported on the outer part of the catalyst carrier in which the additive is present can perform. Further, the noble metal supported on the portion of the carrier such as alumina, zirconia and titania in which the additive is not present not only is wasteful but also can cause methanization reaction which is a side reaction.
In order to obtain a sufficient catalyst activity or reaction selectivity, it is necessary that a large amount of a noble metal be supported on the catalyst or the catalyst be used at low temperatures where methanization reaction which is a side reaction can occur little but the reaction rate is lowered.
In the case where Pt is supported on ceria, ceria reacts with carbon dioxide or water in the modified gas to produce carbonates which then cover the surface of Pt, causing the deterioration of the catalyst activity. However, in the case where Pt is supported on a cerium-zirconium composite oxide, zirconium, which is a stable element, inhibits the deterioration of the catalyst activity.
However, such a cerium-zirconium composite oxide may have a slight amount of an amorphous free ceria crystal present therein even when observed by X-ray diffractometry to have a single phase crystal formed therein. This is because ceria which has been left undissolved in solid state in the cerium-zirconium composite oxide during the preparation of the cerium-zirconium composite oxide separates out on the surface of the cerium-zirconium composite oxide.
The inventors noticed that ceria which has thus separated out can react with carbon dioxide or water in the modified gas to produce carbonates which then cover the surface of Pt to lower the catalyst activity.
Therefore, there is a problem that CO removal catalysts for use in fuel cell electricity generation leave something to be desired in properties.
It is an aim of the invention to provide a CO removal catalyst having improvements in catalyst properties for use in fuel cell electricity generation, a method of producing such a CO removal catalyst, a hydrogen purifying device and a fuel cell system in the light of the aforementioned problems with the related art technique.